1. Field of the Invention
The present invention relates to a method of mounting a plurality of electronic parts on a circuit board, and also to a method of producing adhesive-coated electronic parts suited for use in the mounting method.
2. Description of the Related Art
With a recent trend to smaller-sized, thinner electronic parts such as semiconductor chips (xe2x80x9celectronic partsxe2x80x9d referred to herein include resistors, capacitors, semiconductor chips, etc. mounted on circuit boards), circuits and electrodes used in such electronic parts have increased density and finer connection pitch. Since fine electrodes are difficult to connect by soldering, recently, connection methods using adhesive are widely used. The connection methods include a method in which electrically conducting particles are mixed in an adhesive, and contact bonding is performed to achieve electrical connection in the thickness direction of the adhesive (e.g., Unexamined Japanese Patent Publication (KOKAI) No. 55-104007). There is another and a method in which no conducting particles are contained in an adhesive, and contact bonding is performed to achieve electrical connection through direct contact of fine irregularities on the electrode surfaces (e.g., Unexamined Japanese Patent Publication (KOKAI) No. 60-262430).
The connection methods using adhesive permit connection at relatively low temperatures and also provide excellent reliability because the interconnecting portion has flexibility. In addition, in the case where adhesive formed into a film or tape is used, it is possible to supply the adhesive of uniform thickness in the form of a long strip, whereby the mounting line can be automated. Also, a simple step of applying heat and pressure simultaneously attains electrical connection between electrodes of the semiconductor chip and the circuit board and mechanical connection of the two through bonding. This is why the connection methods using adhesive are attracting attention.
In recent years, multi-chip modules (MCM) which employ a more elaborate form of the above methods and in which a large number of chips, are mounted at high density on circuit boards of relatively small size are drawing attention. In general, an MCM is fabricated by forming an adhesive layer on a circuit board, peeling a separator, if any, from the adhesive layer, and positioning chips such that their electrodes face corresponding electrodes on the circuit board, followed by bonding of the electrodes. Forming an adhesive layer on a chip instead presents the problem that a complicated apparatus is required because a chip having a smaller area than the circuit board needs to be applied with an adhesive layer.
Electronic parts used in MCM include a variety of chips such as semiconductor chips, active elements, passive elements, resistors and capacitors.
Thus, various types of chips having different sizes (areas, heights) are mounted on MCM. When connecting chips to a circuit board, however, a problem arises which is not associated with conventional techniques such as the method of forming an adhesive layer on a circuit board or the heat-pressure bonding method.
Specifically, in the case where the adhesive used is in the form of a film, adhesive strips (adhesive tapes) with different widths are needed depending on different chip sizes. In MCM, however, multiple chips are mounted at high density on a small-sized circuit board, and thus only a small mounting space is available, making it difficult to use a variety of tape widths. Also, use of various tape widths increases the labor involved in the management of materials. Further, since different devices for feeding, contact bonding, tape rewinding, etc. are needed for individual tape widths, the mounting apparatus is inevitably increased in overall size and is complicated, requiring a large installation space and increasing the cost.
An attempt has therefore been made to mount various sizes of chips after an adhesive layer is formed on the entire surface of a circuit board (Examined Japanese Patent Publication (KOKOKU) No. 61-27902). With this method, however, much labor is required to remove the remaining adhesive from non-connecting sections, and also the cost increases because the adhesive layer is formed uselessly on regions other than the mounting sections. Further, since the adhesive is applied to the entire surface of the circuit board, heat applied at the time of connection can adversely affect adjacent chip mounting sections. For example, the reaction of the thermosetting adhesive may progress to such an extent that the adhesive on an adjacent section where a chip is not yet mounted becomes unusable, or an adjacent chip may develop a connection defect as the adhesive softens due to the connection heat even after the chip is mounted. This is also the case with removal of a defective chip after chip mounting. Namely, it is difficult to peel off a defective chip and also to remove the adhesive because of the reaction of the thermosetting adhesive.
Also, as an attempt to form an adhesive layer with a size substantially equal to the chip size, Examined Japanese Patent Publication (KOKOKU) No. 4-30742, for example, discloses forming an adhesive layer on a wafer and then subjecting the wafer to full dicing. In this case also, various types of adhesive-coated wafers must be prepared for different types of chips, making the process control complicated in view of the shelf stability life of the adhesive.
Unexamined Japanese Patent Publications (KOKAI) No. 63-276237 and No. 2-199847, for example, disclose applying an adhesive only to the top faces of bump electrodes (also merely called bumps) on a chip, in order to reduce the connectable pitch. However, since the adhesive is applied only to the top faces of the bump electrodes, the bump electrodes are bonded to a circuit board only in areas around the bump electrodes, so that the bonding strength and the connection reliability are low. In order to also apply the adhesive to regions other than the top faces of the bump electrodes, an underfill material needs to be poured, which, however, complicates the process and increases the cost.
Further, in the case where chips with different heights are mounted or chips are mounted on both surfaces of a circuit board, heat and pressure cannot be uniformly applied by using conventional techniques generally employed, such as a press method in which a chip-carrying board is clamped by parallel mold elements or a pressure roll method using parallel rolls. Thus it is impossible to connect fine electrodes in this situation.
The present invention was created to eliminate the above-described drawbacks, and an object thereof is to provide a method of efficiently mounting electronic parts on a circuit board, groups of adhesive-coated electronic parts suited for use in the mounting method, and a method of producing adhesive-coated electronic parts.
According to a first aspect of the present invention, there is provided a method of mounting a plurality of electronic parts on a circuit board by bonding and fixing electrodes of the electronic parts to the circuit board to electrically connect the individual electronic parts to the circuit board. The method comprises: an adhesive layer formation step of forming, on an electrode surface of each of the electronic parts on which the electrodes are formed, a film-like thermosetting adhesive layer having an area substantially equal to that of the corresponding electrode surface, to obtain adhesive-coated electronic parts; a positioning step of arranging the electrodes (on which the adhesive layer is formed) so as to face corresponding electrodes of the circuit board and positioning the electrodes relative to each other; and a heat-pressure bonding step of applying heat and pressure to the electrodes of the electronic parts and the electrodes of the circuit board to fix the electrodes to each other after the electrodes are positioned.
According to the first aspect of the invention, the adhesive layer is formed beforehand on the electrode surface of each electronic part the electrode surface thus applied with the adhesive is affixed to corresponding electrodes on the circuit board, so that almost no adhesive superfluously comes out of the electrode surfaces. Accordingly, when mounting the electronic parts on the circuit board, it is unnecessary to remove superfluous adhesive, unlike the conventional process, whereby the efficiency is improved and the cost can be reduced.
The electrodes are bonded to the circuit board with heat and pressure applied thereto after the electrodes are set in position, and therefore, the electrodes can be shifted as needed and thus can be positioned with accuracy. Even in the case where electronic parts with different heights or sizes are mounted, the electrodes of the electronic parts are individually fixed with heat and pressure applied thereto, whereby the electrodes can be applied uniformly with heat and pressure and the electronic parts can be easily mounted with reliability. In particular, it is possible to connect fine electrodes.
Preferably, the area of the film-like adhesive layer falls within a range of xc2x130% with respect to the area of the electrode surface of the corresponding electronic part. If the area of the adhesive layer is greater than the xc2x130% range, too much adhesive comes out of the electrode surfaces, possibly requiring the adhesive removing step; on the other hand, if the area of the adhesive layer is smaller than the xc2x130% range, then there is the possibility of the electronic parts failing to be satisfactorily connected.
The electronic parts are preferably held by a heating head by means of suction, for example, so that the surfaces of the electronic parts can be heated by the heating head. The heating head serves to locate the electronic parts in predetermined position while holding the same, and then to immediately heat the electronic parts to be bonded and fixed to the circuit board. Thus the apparatus and the process can be simplified.
The heat-pressure bonding step preferably includes an inspection step of inspecting the electrical connection between the electrodes while the cohesive strength of the adhesive is increased to such an extent that the connection of the electrodes can be maintained. Namely, while the electronic parts are temporarily fixed with the cohesive force of the adhesive increased, the electrical connection is inspected. Even in the case where defective connection is discovered, repair work can be easily carried out because the electronic parts are fixed only temporarily.
According to a second aspect of the present invention, there is provided a method of mounting a plurality of electronic parts on a circuit board by bonding and fixing electrodes of the electronic parts to the circuit board to electrically connect the individual electronic parts to the circuit board. The method comprises: an adhesive layer formation step of forming, on an electrode surface of each of the electronic parts on which the electrodes are formed, a film-like thermosetting adhesive layer having an area substantially equal to that of the corresponding electrode surface, to obtain adhesive-coated electronic parts; a temporary fixing step of positioning the electrodes of the electronic parts (on which the adhesive layer is formed) so as to face corresponding electrodes of the circuit board, and increasing cohesive strength of the adhesive to such an extent that connection of the electrodes can be maintained; and a heat-pressure bonding step of applying heat and pressure to the temporarily fixed electrodes to fix the electrodes to each other, the heat-pressure bonding step including simultaneously heating a plurality of electronic parts in an autoclave with a static pressure applied thereto within the autoclave.
According to the second aspect of the invention, in the heat-pressure bonding step according to the first aspect of the invention, a plurality of electronic parts are simultaneously heated in the autoclave under the static pressure within the autoclave. Thus multiple electronic parts can be easily bonded and fixed at one time with a simple arrangement.
Also in the second aspect of the invention, the area of the film-like adhesive layer preferably falls within a range of xc2x130% with respect to the area of the electrode surface of the corresponding electronic part, as mentioned above.
Further, the heat-pressure bonding step preferably includes an inspection step of inspecting the electrical connection between the electrodes while the cohesive strength of the adhesive is increased to such an extent that the connection of the electrodes can be maintained. Namely, while the electronic parts are temporarily fixed with the cohesive strength of the adhesive increased, the electrical connection is inspected. Even in the case where a defective connection is discovered, repair work can be easily carried out because the electronic parts are fixed only temporarily.
According to a third aspect of the present invention, there is provided a group of adhesive-coated electronic parts each having an electrode surface coated with a film-like adhesive layer. The adhesive-coated electronic part group comprises: a connection sheet including a film-like adhesive layer and a separator from which the film-like adhesive layer can be peeled; and a plurality of electronic parts arranged on the film-like adhesive layer of the connection sheet, each of the electronic parts being affixed to the adhesive layer at an electrode surface thereof.
According to the third aspect of the invention, a plurality of electronic parts are affixed to the adhesive film. Therefore, when the electronic parts are used, they are peeled off the separator film is peeled off, so that the electrode surface of each electronic part is coated with the adhesive layer. Namely, adhesive-coated electronic parts can be readily obtained, and in the case where electronic parts are mounted on circuit boards, a connection sheet with electronic parts affixed thereon is prepared and the electronic parts are peeled off of the connection sheet, thereby obtaining adhesive-coated electronic parts ready for use. Consequently, the process control of the mounting process is facilitated, and also the adhesive-coated electronic parts have excellent shelf stability.
Preferably, the adhesive layer contains electrically conducting particles. The conducting particles serve to electrically connect electrodes facing each other with reliability and also to insulate adjacent electrodes from each other.
According to a fourth aspect of the present invention, there is provided a method of producing an adhesive-coated electronic part having an electrode surface coated with a film-like adhesive layer. The method comprising a connection sheet placement step of arranging a connection sheet including a film-like adhesive layer and a separator from which the adhesive layer can be peeled, the connection sheet having a size greater than that of an electrode surface of an electronic part on which electrodes are formed; a contacting step of bringing the electrode surface of the electronic part into contact with the adhesive layer; a heating step of heating the electrode surface to form a cohesion reduction line at which cohesive strength of the adhesive lowers, at a location between a region of the adhesive layer corresponding to the electrode surface and a region of the adhesive layer surrounding the electrode surface; and a separating step of separating the electronic part from the connection sheet such that part of the adhesive layer having a size substantially identical with that of the electrode surface is separated from the separator and adheres to the electrode surface.
According to the fourth aspect of the invention, the electrode surface of the electronic part is brought into contact with the connection sheet, and the electrodes are heated so that the cohesive strength of the adhesive around the electrodes may lower. Namely, the adhesive sets with a certain cohesive strength at normal temperature, but the cohesive strength lowers (or the adhesive softens) when the adhesive is heated up to a predetermined temperature. Accordingly, when the electronic part is separated from the connection sheet, part of the adhesive layer corresponding in size to the periphery of the electrode surface separates from the separator and adheres to the electrode surface, thus easily obtaining an adhesive-coated electronic part which is coated with an adhesive layer having a size substantially equal to that of the electrode surface.
The cohesion reduction line is formed along the periphery of the electrode surface and is the boundary between a region of the adhesive layer where the cohesive strength lowers when the electrodes are heated and a region of the adhesive layer where the cohesive strength remains almost the same. In the case of adhesive, cohesive property is different from setting or hardening property (activation). Thus the former term is applicable to both thermosetting adhesive and heat softening adhesive.
According to a fifth aspect of the invention, there is provided a method of producing an adhesive-coated electronic part having an electrode surface coated with a film-like adhesive layer. The method comprises: a connection sheet placement step of arranging a connection sheet including a film-like adhesive layer and a separator from which the adhesive layer can be peeled, the connection sheet having a size greater than that of an electronic part; a contacting step of bringing an electrode surface of the electronic part on which electrodes are formed into contact with the adhesive layer; a cutting step of pressing the electrode surface against the adhesive layer and cutting at least part of the adhesive layer along a periphery of the electronic part; and a separating step of separating the electronic part from the connection sheet such that part of the adhesive layer is separated from the separator and adheres to the electrodes.
According to the fifth aspect of the invention, the electrode surface of the electronic part is pressed against the adhesive layer of the connection sheet, and the adhesive layer is cut along the electrode surface (electronic part) to thereby separate part of the adhesive layer which is in contact with the electrode surface from the other part of the adhesive layer. Consequently, it is possible to easily obtain with reliability an adhesive layer having a size substantially identical to that of the electrode surface of the electronic part.
Preferably, the cutting step is achieved using a cutter arranged at a pressure head for pressing the electrodes of the electronic part against the connection sheet, or using a heating wire. Using the cutter arranged at the pressure head or the heating wire makes it possible to easily obtain an adhesive layer having a size substantially equal to that of the electronic part or the electrode surface.
Preferably, the connection sheet is placed on a surface plate with a cushioning member interposed therebetween. In this case, the impact at the time of application of pressure by the pressure head or at the time of cutting can be absorbed.
Further, the adhesive layer preferably contains electrically conducting particles in order to enhance the insulating property of an electrode from adjacent electrodes, as mentioned above.